Due to the deployment of more and more frequency bands for radio-communications applications, the complexity of radio front-end circuitry to be used in such applications increases. Normally, at least one relatively expensive external (or “off-chip”) filter, usually a SAW (Surface Acoustic Wave) filter and/or duplexer, has to be used for each frequency band to be received with the cellular radio. Therefore the size and cost of the external front-end components increase when introducing new frequency bands. Striving towards more flexible front-end solutions requires new circuit solutions that can handle strong interferers and prevent harmonic down-conversion without sacrificing any other performance.
It is thus desirable to provide radio front-end circuitry that eliminates the need for at least some of the off-chip filters, e.g. relatively expensive SAW filters and/or duplexers, that are normally used in present cellular radio communications circuitry, or that at least relaxes the requirements on such off-chip filters, which in turn facilitates a reduction of manufacturing cost.
To handle the strong out-of-band interference in cell phones, or other similar communication devices, without using sharp radio frequency (RF) filters, such as SAW filters and/or duplexer, a relatively high linearity is normally needed. Otherwise the unfiltered amplified interference might saturate low-noise amplifiers (LNAs) or down-conversion mixers. To reduce the interference, it has been proposed (e.g. in US 2005/0239430 A1) to use so called harmonic rejection in down-conversion circuitry to suppress interference at harmonics of a local oscillator (LO) signal, which is particularly useful in a radio receiver without sharp RF filters, since interferers at harmonics of the LO signal will be down-converted to baseband and, unless suppressed in some way, detrimentally interfere with the (desired) baseband signal.